How to get refresh token in google

Question

I'm getting an access token and that access token expiry time is 1 hour but after few event creation access token getting expired that's why i want's to store refresh token and using refresh token i can re-generate new access token but i'm unable to get refresh token.

Token Uri : "https://oauth2.googleapis.com/token"

Request data:

{
    "redirect_uri": "www.myapp.com",
    "grant_type": "authorization_code",
    "client_id": "873581********8******apps.google*****.com",
    "code": "{{code}}",
    "client_secret": "GOCSPX-r****ci_i8ii****iD",
    "access_type": "offline"
}

Response:

{
  "access_token" : "HUGE_TOKEN_STRING",
  "token_type" : "Bearer",
  "expires_in" : 3600
}